Shear Strength of Wood Beams

The relationship between shear strength of Douglas-fir glued-laminated timber beams and shear span-to-depth ratio (a/d) is not satisfactorily explained either by presence of compressive stresses perpendicular to grain, or by “two- beam” action. When a shear failure in dry Douglas-fir is free to seek its plane of greatest weakness–as is usually the case in glued-laminated beams–its shear strength was not found to increase significantly with application of compressive stress perpendicular to grain. At the same time, finite-element stress analyses of unchecked Douglas-fir glued laminated beams showed shear stress distributions for most of the a/d domain in question about the same as those predicted by ordinary mechanics, with no indication of “two-beam” action. Shear strength of dry Douglas-fir, no matter how it is deter mined, appears to be related to “sheared area” As. As is the product of the beam width and shear span. It is suggested that the a/d effect is actually just one manifestation of this over all size effect. Since “two-beam” action does not appear to apply to Douglas-fir glued-laminated timber beams, consider ation should be given to modifying design codes to reflect this. In fact, assuming “two-beam” action in conjuction with large As values may lead to unsafe situations, while members with low As values may be designed too conservatively.

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